The present invention relates to a reflection-type image projection unit or device for projecting an image therefrom, using an image display element therein, such as a reflection-type liquid crystal panel, for example, further a reflection-type image projection display apparatus using such an image projection device therein, and further it relates also to a light source device for use therein.
Conventionally, various types or methods are already known for a video or image display apparatus of the reflection-type, in which light from a light or optic source of high brightness, such as a metal halide lamp or a super high-pressure mercury lamp, etc., for example, are irradiated upon an image display element, such as the reflection-type liquid crystal, for example, thereby projecting the reflected light upon, for example, a screen, etc., through a projection optical system, which is made up with a projection lens, etc., and also various improvements and proposals are made for it.
First of all, for example in Japanese Patent Laying-Open No. 2000-81603 (JP-A 81603/2000) is already known a projection display system for use with a reflection-type liquid crystal panel, wherein after projection of the light upon LCDs (i.e., the reflection-type liquid crystal panel elements), being emitted from the optic source portion and divided into three (3) components (R,G,B) being different in the wavelengths, the reflected light from those LCDs are optically synthesized or combined using two (2) pieces of dichroic mirrors and two (2) pieces of plate-like beam splitters, thereby projecting those through the projection lens onto a wide display, such as an image screen, for example.
However, in particular, since the liquid crystal projector apparatus or device of rear-projection-type must be thin in the thickness, it is desirable that the size of the apparatus as a whole be small when it is used as a projection unit. From the viewpoint of small-sizing of such a device, in order to make an optical system small, to shorten the optical path thereof, and further to improve an accuracy in manufacturing of the optical system, as was shown or disclosed in Japanese Patent Laying-Open No. Hei 11-153774 (JP-A153774/1999) or Japanese Patent Laying-Open No. Hei 11-326834 (JP-A 326834/1999), for example, a reflection-type light modulation projector was already provided or known, in which the light of three (3) color components reflected upon the three (3) reflection-type image display elements are optically combined or synthesized by means of a dichroic mirror of a prism-type (i.e., a dichroic prism) or a polarization beam splitter (PBS).
Additionally, for example, in Japanese Patent Laying-Open No. 2000-284228 (JP-A284228/2000), for achieving the small-sizing of the liquid crystal projector apparatus, in particular for solving large-sizes of the apparatus due to the disposition of the conventional art, i.e., the dichroic prism and three (3) pieces of the prism-type PBSs, surrounding it in three (3) directions thereof, there is proposed the structure, in which four (4) pieces of the prism-type dichroic mirrors (i.e., the dichroic prisms) and the prism-type polarization beam splitters (i.e., PBSs) are disposed in a block-like manner, building up a color division portion, a GB modulator portion, an R modulator portion and a color synthesizer portion, respectively, and further in which the optical path is prevented from turning around in excess by using a narrow bandwidth phase (difference) plate, thereby enabling the small-sizing of the apparatus.
Furthermore, for example, in Japanese Patent Laying-Open No. 2001-154294 (JP-A 154294/2001) are proposed various constructions of the image display device, in details, for achieving the small-sizing, light-weighting, as well as, low-cost, brightness and high picture quality, and herein are provided various structures of an optical engine, wherein an optical image is formed or synthesized from the light emitted from the optical source, corresponding to an image signal, and thereby emitting the reflected light synthesized.
However, with such the conventional arts mentioned above, in particular as is representatively shown in the Japanese Patent Laying-Open No. 2000-81603 (JP-A81603/2000) mentioned above, i.e., with such the method of obtaining the optical synthesizing by using plural plate-like dichroic filters and plate-like polarization beam splitters, since a space within the optical path is not filled up with a medium (though being filled up with the air), the back-focus is long when seeing a light division/synthesizing portion from the projection lens, therefore it is difficult to make compensation or calibration on the focusing capacity and/or distortion, etc. Also, since the light beam emitted from the optic or light source has an extent or expanse, the structure still is insufficient for obtaining the small-sizing of the apparatus. Further, since it is large in the optical system thereof, it is also difficult to be manufactured through assembling with high accuracy.
Also, with such the structure of the projection-type image display apparatus, as was already known from Japanese Patent Laying-Open No. Hei 11-153774 (JP-A153774/1999) or Japanese Patent Laying-Open No. Hei 11-326834 (JP-A 326834/1999) mentioned above, and further the Japanese Patent Laying-Open No. 2001-154294 (JP-A 154294/2001) mentioned above, indeed, comparing to the structure built up by using the plural plate-like dichroic filters and the polarization beam splitters known by the Japanese Patent Laying-open No. 2000-81603 (JP-A 81603/2000) mentioned above, it is able to achieve the small-sizing of the apparatus, however with the picture quality obtainable on the apparatus, it still has the following problems, in particular relating to the capacity of contrast.
Namely, in the projection-type image display apparatus, generally the light divided in the color dividing portion forms an optical image in a G modulator portion, a B modulator portion and a R modulator portion, each using the image display element, respectively, and thereafter they are synthesized with and projected onto the image screen through the projection lens, so as to obtain an image thereon. In this instance, with the photo-elastic constant or ratio of the material making up the opt-synthesizing optical system therefrom, it is possible to obtain an expanded projection image with less color shading and/or shading in brightness, by combining the materials, respectively, being different in the wavelength at which the photo-elastic constant lies at the minimum depending upon the optical path for the color light beams to be synthesized and/or the relative visibility thereof. In particular, with the contrast, it is possible to obtain a preferable expanded projection image with using the material being smallest in the photo-elastic constant with respect to a green color light (G), which shows the largest value in the relative visibility. Also, in particular, the polarization light beam splitter (PBS) of the prism-type, for making up the light dividing/synthesizing portion, shows the great difference in the characteristics thereof, in particular in the case when it is used in the light dividing/synthesizing portion under the condition of penetrating the light beam therethrough, and in the case when it is used under the condition of reflecting it thereupon. And also, since an illumination light has an angle component for a F value, an amount of light leakage differs greatly, in the case where it is used under the condition of reflection and in the case where it is used under the condition of penetration, and therefore the difference is generated in the attenuation factor thereof. However with those conventional arts mentioned above, it is still under the situation at the present, that no proposal is made for the light dividing/synthesizing optical system, in which the consideration is fully paid on the problems, inherent or unique to those structural parts or elements thereof.